The invention relates to the cathode sputtering of metals in the presence of a reactive gas to form a durable, transparent coating on a substrate.
Windows of modern buildings and vehicles are provided with coatings having particular optical properties. Thin metal films are deposited on glass or plastic to increase the reflectance of solar radiation. Even more energy efficient windows include a multilayer dielectric-metal-dielectric coating having a high visible transmittance combined with high reflectivity and low emissivity in the infrared wavelength range of the electromagnetic spectrum. The index of refraction of the dielectric layer is preferably 2.0 or greater in order to minimize the visible reflectance and enhance the visible transmittance of the window. Such multilayer coatings are frequently deposited by sputtering metal targets in an atmosphere of inert gas to deposit a metal layer and in an atmosphere containing a reactive gas to deposit the dielectric layers. Such sputtered coatings are described in U.S. Pat. No. 4,610,771 to Gillery.
One such dielectric-metal-dielectric coating comprises successive layers of zinc oxide, silver, zinc and zinc oxide deposited on a float-glass substrate. Silver is preferred because of its high infrared reflectance, but the metal layer may also be gold, copper, or another good electrical conductor. When silver is used, it is usually covered with a very thin layer of zinc, tin, titanium, aluminum or other metal to protect the silver from corrosion. Zinc oxide is a preferred dielectric because it has a refractive index of 2.0 and it can be deposited by reactive sputtering at rates which are relatively high for oxides. Various other materials, such as oxides of bismuth, titanium, tin and indium, and nitrides of aluminum are also used as dielectric layers.
Usually the coated glass is sealed as part of a double-glazed window unit in which the coating lies on an inside surface where the coating is protected by glass from abrasion and environmental materials which could cause corrosion and degrade the optical properties of the coating. Abrasion and corrosion are problems even while handling of the coated glass before and during the fabrication process.
There is a recognized need for a harder and more corrosion resistant dielectric material to provide a protective overcoat for metal or dielectric coatings or to replace some or all of one or both of the dielectric layers in a dielectric-metal-dielectric coating. The present invention relates to the use of reactively sputtered aluminum-silicon alloys for these and other purposes.
Aluminum-silicon alloys have previously been sputtered in a reactive gas.
In a paper "Preparation of Thin Mullite Films", J. Am. Ceram Soc., Vol. 46, pp. 161-167 (1963), J. C. Williams et al, disclosed deposition of mixed Al.sub.2 O.sub.3 /SiO.sub.2 films by sputtering in oxygen of aluminum-silicon alloy targets having 20, 28 and 40 wt. % silicon.
In a paper, "Influence of Residual Gases on the Properties of DC Magnetron-Sputtered Aluminum-Silicon", J. Vac. Sci. Technol., Vol. 17, pp. 384-387 (1980), R. S. Nowicki disclosed metal films deposited by sputtering a target of aluminum-1.5% silicon in gas of argon-1% oxygen.